Always start python via shebang lines.

This is preparation for supporting using Python 3 in production.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

contrib_bots/lib/readme.md

@@ -30,7 +30,7 @@ Here is an example of running the "follow-up" bot from
 inside a Zulip repo:
 
     cd ~/zulip/contrib_bots
-    python run.py lib/followup.py --config-file ~/.zuliprc-prod
+    ./run.py lib/followup.py --config-file ~/.zuliprc-prod
 
 Once the bot code starts running, you will see a
 message explaining how to use the bot, as well as

contrib_bots/run.py

@@ -1,3 +1,4 @@
+#!/usr/bin/env python
 from __future__ import print_function
 
 import importlib
@@ -71,9 +72,9 @@ def run_message_handler_for_bot(lib_module, quiet, config_file):
 
 def run():
     usage = '''
-        python run.py <lib file>
+        ./run.py <lib file>
 
-        Example: python run.py lib/followup.py
+        Example: ./run.py lib/followup.py
 
         (This program loads bot-related code from the
         library code and then runs a message loop,